This invention relates generally to flash tube type electric discharge lamps and more particularly to lamps of this type which are operated at reasonably high energy loadings for extended time periods up to one million flashes and greater. Such type flash tubes generally comprise a pair of spaced apart electrodes within the sealed light-transmitting envelope having a rare gas filling, typically xenon, at subatmospheric pressures. Said discharge lamps further customarily include an auxiliary surface electrode to initiate the xenon discharge and which can be provided with a transparent metal oxide coating, such as tin oxide, being applied to the exterior surface of the sealed light-transmitting envelope. While the original light-transmitting envelopes were customarily formed from fused quartz, other glass materials including aluminosilicate glass and borosilicate glass compositions have been known for such use as well as even soft glasses if the particular lamp operating conditions are adjusted to reduce environmental stress on said glass member.
Environmental problems most frequently experienced in flash tube operation are thermal stress which can understandably lead to premature lamp failure and solarization of the glass envelope material which can lead to early discoloration with a substantial light loss over the lamp lifetime. A further problem encountered during lamp manufacture is reliability of the hermetic seal between the lamp glass material and metal alloys used to form the lamp electrical inleads. Specifically, there is a tendency for said glass-to-metal seals to be unreliable unless the thermal expansion characteristics of both materials are closely matched and any stones, knots or non-homogeneity in the glass material further tends to cause the seal to be rejected for appearance, dimensional non-conformity, or structurally weak areas. A still further problem which can impair reliable formation of a direct glass-to-metal seal is undesirable chemical reaction between the metal alloy and the glass material during the sealing procedure. Metal oxides in the glass material can be macroscopically reduced by the metal elements in the inlead alloys as a further means of contributing to seal failure or mechanical weakness in the seal. By reason of all the foregoing problems, borosilicate glass materials have not yet been employed commercially to any significant degree in flash tube constructions.